Many antibiotics are already known, but new antibiotic substances are still wanted to be provided in the pharmaceutical field and also in the agricultural field. We, the present inventors, have made extensive researches to discover and provide new antibiotic substances which can exhibit useful antibacterial activity and/or antifungal activity. As a result, we have now found that when a microbial strain of the genus Actinomycetes, which was isolated from a soil sample collected out of the ground in our laboratory in Tokyo, Japan and which was given a laboratory designation of MH193-16F4 strain, is cultivated in a culture medium under aerobic conditions, there are produced some antibiotics which exhibit antifungal activities. We have succeeded in isolating and purifying two antifungal antibiotics from the culture of the MH193-16F4 strain, and we have nominated these isolated two antibiocits as benanomicin A and benanomicin B, respectively. We have studied the physicochemical and biological properties of benanomicins A and B to confirm that benanomicins A and B are new substances distinguishable from any of the known antibiotics. Through our further study, we have now succeeded in deciding the chemical structures of benanomicins A and B. Furthermore, we have now succeeded in synthetizing a new compound, namely dexylosylbenanomicin B by a chemical conversion of benanomicin B and have found that dexylosylbenanomicin B also can exhibit a useful antifungal activity.
We have further found that the new antibiotics, benanomicins A and B according to this invention are more or less similar in their physicochemical and biological properties as well as in their chemical structures to known three antibiotics, namely KS-619-1 substance {Matsuda et al: the "Journal of Antibiotics" 40, 1104-1114 (1987)}; and G-2N substance and G-2A substance {Gerber et al: the "Canad. J. Chem." 62, 2818-2821 (1984)}, but that benanomicins A and B can clearly be differentiated from the above-mentioned known three antibiotics in view of their physicochemical and biological properties and also their chemical structures.
Hithertobefore, a variety of antibotics which are produced by microorganisms are already known. Among the known antibiotics, however, such antibiotics which can exhibit a useful antifungal activity but a low toxicity to mammals are only few. Accordingly, there is always a demand for discovery and exploitation of a new antifungal antibiotic which is useful in the therapeutic treatment of various fungal infections in an animal, including human. We have now found that benanomicins A and B as well as dexylosylbenanomicin B are of a low toxicity to an animal and that they can be represented by a general formula (I) shown hereinafter.
Therefore, an object of this invention is to provide new antibiotics which can exhibit a usefully high antifungal activity with a low toxicity to mammals, and more particularly benanomicin A, benanomicin B and dexylosylbenanomicin B as well as their pharmaceutically acceptable salts and esters. Another object of this invention is to provide processes for the production of these new antibiotics. Further objects of this invention will be apparent from the following descriptions.